The present invention relates to an illumination apparatus for use in an image-taking apparatus or the like.
An illumination apparatus for use in an image-taking apparatus such as a camera is formed of a light source and optical members such as a reflective mirror and a Fresnel lens for directing luminous flux emitting from the light source toward a light irradiation side (toward a subject). A variety of small illumination apparatuses have been proposed for efficiently gathering luminous flux emitting from the light source in various directions to a necessary irradiation area.
Particularly, some of recently proposed apparatuses employ an optical member which achieves total reflection such as a prism and a light guide provided to realize uniform light distribution, improved light-gathering efficiency, and a reduced size, instead of a Fresnel lens disposed closer to a subject than a light source.
For example, Japanese Patent Laid-Open No. 2000-250102 has proposed an illumination apparatus having an optical prism for applying luminous flux from a light source in a light irradiation direction, in which the optical prism has a first entrance surface which receives part of the luminous flux from the light source that emits close to an irradiation optical axis, an emergence surface from which the luminous flux from the first entrance surface directly emerges, a second entrance surface which receives part of the luminous flux from the light source that emits at a larger angle than that close to the irradiation optical axis, and a total reflection surface which totally reflects the luminous flux from the second entrance surface and causes the reflected luminous flux to emerge from the emergence surface. In the illumination apparatus, the abovementioned respective surfaces are formed to have shapes in which a certain correlation is established between the angle of luminous flux emitting from the center of the light source with respect to the irradiation optical axis and the angle of luminous flux emerging from the emergence surface with respect to the irradiation optical axis.
As the material of the optical prism, an optical resin material such as acrylic resin is often used in view of the moldability and cost.
In the illumination apparatus of this type, the light source produces a large amount of heat together with light. To prevent deformation of the optical material due to the heat, it is necessary to select the optical material and define the heat radiation space in consideration of thermal energy produced in one light emission and the shortest light-emission cycle. As a result, in the illumination apparatus proposed in Japanese Patent Laid-Open No. 2000-250102 described above, a somewhat large distance needs to be ensured between the second entrance surface which receives the luminous flux emitting from the light source at a relatively large angle and the light source.
With the position of the second entrance surface set in this manner, however, the height of the optical prism is easily increased, which makes it difficult to reduce the size of the illumination apparatus.